This invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays, and to apparatus (controllers) for use in such methods. More specifically, this invention relates to driving methods which are intended to enable more accurate control of gray states of the pixels of an electro-optic display. This invention also relates to driving methods which are intended to enable such displays to be driven in a manner which allows compensation for the “dwell time” during which a pixel has remained in a particular optical state prior to a transition, while still allowing the drive scheme used to drive the display to be DC balanced. This invention is especially, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are suspended in a liquid and are moved through the liquid under the influence of an electric field to change the appearance of the display.
The electro-optic displays in which the methods of the present invention are used often contain an electro-optic material which is a solid in the sense that the electro-optic material has solid external surfaces, although the material may, and often does, have internal liquid- or gas-filled space. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as “solid electro-optic displays”.
The term “electro-optic” as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all. The term “gray level” is used herein to denote the possible optical states of a pixel, including the two extreme optical states.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “impulse” is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
Much of the discussion below will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term “waveform” will be used to denote the entire voltage against time curve used to effect the transition from one specific initial gray level to a specific final gray level. Typically, as illustrated below, such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., there a given element comprises application of a constant voltage for a period of time), the elements may be called “voltage pulses” or “drive pulses”. The term “drive scheme” denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. No. 6,301,038, International Application Publication No. WO 01/27690, and in U.S. Patent Application 2003/0214695. This type of medium is also typically bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also European Patent Applications 1,429,178; 1,462,847; and 1,482,354; and International Applications WO 2004/090626; WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO 2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO 03/088495. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a fluid, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; and 6,922,276; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0180687; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0214695; 2003/0222315; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0001812; 2005/0007336; 2005/0007653; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0035941; 2005/0041004; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0105159; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0151709; and 2005/0152022; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 03/107,315.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within capsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and U.S. Patent Application Publication No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Other types of electro-optic media may also be used in the displays of the present invention.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior (such displays may hereinafter for convenience be referred to as “impulse driven displays”), is in marked contrast to that of conventional liquid crystal (“LC”) displays. Twisted nematic liquid crystals act are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.
Whether or not the electro-optic medium used is bistable, to obtain a high-resolution display, individual pixels of a display must be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel, to produce an “active matrix” display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the “line address time” the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed to that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner.
It might at first appear that the ideal method for addressing such an impulse-driven electro-optic display would be so-called “general grayscale image flow” in which a controller arranges each writing of an image so that each pixel transitions directly from its initial gray level to its final gray level. However, inevitably there is some error in writing images on an impulse-driven display. Some such errors encountered in practice include:                (a) Prior State Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical state, but also on the previous optical states of the pixel.        (b) Dwell Time Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends on the time that the pixel has spent in its various optical states. The precise nature of this dependence is not well understood, but in general, more impulse is required that longer the pixel has been in its current optical state.        (c) Temperature Dependence; The impulse required to switch a pixel to a new optical state depends heavily on temperature.        (d) Humidity Dependence; The impulse required to switch a pixel to a new optical state depends, with at least some types of electro-optic media, on the ambient humidity.        (e) Mechanical Uniformity; The impulse required to switch a pixel to a new optical state may be affected by mechanical variations in the display, for example variations in the thickness of an electro-optic medium or an associated lamination adhesive. Other types of mechanical non-uniformity may arise from inevitable variations between different manufacturing batches of medium, manufacturing tolerances and materials variations.        (f) Voltage Errors; The actual impulse applied to a pixel will inevitably differ slightly from that theoretically applied because of unavoidable slight errors in the voltages delivered by drivers.        
General grayscale image flow suffers from an “accumulation of errors” phenomenon. For example, imagine that temperature dependence results in a 0.2 L* (where L* has the usual CIE definition:L*=116(R/R0)1/3−16,where R is the reflectance and R0 is a standard reflectance value) error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10 L*. Perhaps more realistically, suppose that the average error on each transition, expressed in terms of the difference between the theoretical and the actual reflectance of the display is ±0.2 L*. After 100 successive transitions, the pixels will display an average deviation from their expected state of 2 L*; such deviations are apparent to the average observer on certain types of images.
This accumulation of errors phenomenon applies not only to errors due to temperature, but also to errors of all the types listed above. As described in the aforementioned 2003/0137521, compensating for such errors is possible, but only to a limited degree of precision. For example, temperature errors can be compensated by using a temperature sensor and a lookup table, but the temperature sensor has a limited resolution and may read a temperature slightly different from that of the electro-optic medium. Similarly, prior state dependence can be compensated by storing the prior states and using a multi-dimensional transition matrix, but controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, placing a limit on the precision of this type of compensation.
Thus, general grayscale image flow requires very precise control of applied impulse to give good results, and empirically it has been found that, in the present state of the technology of electro-optic displays, general grayscale image flow is infeasible in a commercial display.
Almost all electro-optic medium have a built-in resetting (error limiting) mechanism, namely their extreme (typically black and white) optical states, which function as “optical rails”. After a specific impulse has been applied to a pixel of an electro-optic display, that pixel cannot get any whiter (or blacker). For example, in an encapsulated electrophoretic display, after a specific impulse has been applied, all the electrophoretic particles are forced against one another or against the capsule wall, and cannot move further, thus producing a limiting optical state or optical rail. Because there is a distribution of electrophoretic particle sizes and charges in such a medium, some particles hit the rails before others, creating a “soft rails” phenomenon, whereby the impulse precision required is reduced when the final optical state of a transition approaches the extreme black and white states, whereas the optical precision required increases dramatically in transitions ending near the middle of the optical range of the pixel.
Various types of drive schemes for electro-optic displays are known which take advantage of optical rails. For example, FIGS. 9 and 10 of the aforementioned 2003/0137521, and the related description at Paragraphs [0177] to [0180], describe a “slide show” drive scheme in which the entire display is driven to at least one optical rail before any new image is written. Obviously, a pure general grayscale image flow drive scheme cannot rely upon using the optical rails to prevent errors in gray levels since in such a drive scheme any given pixel can undergo an infinitely large number of changes in gray level without ever touching either optical rail.
Before proceeding further, it is desirable to define slideshow drive schemes more precisely. The fundamental slideshow drive scheme is that a transition from an initial optical state (gray level) to a final (desired) optical state (gray level) is achieved by making transitions to a finite number of intermediate states, where the minimum number of intermediate states is one. Preferably, the intermediate states are at or near the extreme states of the electro-optic medium used. The transitions will differ from pixel to pixel in a display, because they depend upon the initial and final optical states. The waveform for a specific transition for a given pixel of a display may be expressed as:R2goal1goal2 . . . goalnR1  (Scheme 1)where there is at least one intermediate or goal state between the initial state R2 and the final state R1. The goal states are, in general, functions of the initial and final optical states. The presently preferred number of intermediate states is two, but more or fewer intermediate states may be used. Each of the individual transitions within the overall transition is achieved using a waveform element (typically a voltage pulse) sufficient to drive the pixel from one state of the sequence to the next state. For example, in the waveform indicated symbolically above, the transition from R2 to goal1 is typically achieved with a waveform element or voltage pulse. This waveform element may be of a single voltage for a finite time (i.e., a single voltage pulse), or may include a variety of voltages so that a precise goal1 state is achieved. This waveform element is followed by a second waveform element to achieve the transition from goal1 to goal2. If only two goal states are used, the second waveform element is followed by a third waveform element that drives the pixel from the goal2 state to the final optical state R1. The goal states may be independent of both R2 and R1, or may depend upon one or both.
This invention seeks to provide improved slide show drive schemes for electro-optic displays which achieve improved control of gray levels. This invention is particularly, although not exclusively, intended for use in pulse width modulated drive schemes in which the voltage applied to any given pixel of a display at any given moment can only be −V, 0 or +V, where V is an arbitrary voltage. More specifically, this invention relates to two distinct types of improvements in slide show drive schemes, namely (a) insertion of certain modifying elements into base waveforms for such a drive scheme; and (b) arranging the drive scheme so that at least certain gray levels are approached from the optical rail further from the desired gray level.
In another aspect, this invention relates to dwell time compensation in drive schemes for electro-optic displays. As discussed in the MEDEOD applications, it has been found, at least in the case of many particle-based electro-optic displays, that the impulses necessary to change a given pixel through equal changes in gray level (as judged by eye or by standard optical instruments) are not necessarily constant, nor are they necessarily commutative. For example, consider a display in which each pixel can display gray levels of 0 (white), 1, 2 or 3 (black), beneficially spaced apart. (The spacing between the levels may be linear in percentage reflectance, as measured by eye or by instruments but other spacings may also be used. For example, the spacings may be linear in L* or may be selected to provide a specific gamma; a gamma of 2.2 is often adopted for monitors, and when electro-optic displays are be used as a replacement for monitors, use of a similar gamma may be desirable.) It has been found that the impulse necessary to change the pixel from level 0 to level 1 (hereinafter for convenience referred to as a “0-1 transition”) is often not the same as that required for a 1-2 or 2-3 transition. Furthermore, the impulse needed for a 1-0 transition is not necessarily the same as the reverse of that needed for a 0-1 transition. In addition, some systems appear to display a “memory” effect, such that the impulse needed for (say) a 0-1 transition varies somewhat depending upon whether a particular pixel undergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where, the notation “x-y-z”, where x, y, and z are all optical states 0, 1, 2, or 3 denotes a sequence of optical states visited sequentially in time.) Although these problems can be reduced or overcome by driving all pixels of the display to one of the extreme states for a substantial period before driving the required pixels to other states, the resultant “flash” of solid color is often unacceptable; for example, a reader of an electronic book may desire the text of the book to scroll down the screen, and may be distracted, or lose his place, if the display is required to flash solid black or white at frequent intervals. Furthermore, such flashing of the display increases its energy consumption and may reduce the working lifetime of the display. Finally, it has been found that, at least in some cases, the impulse required for a particular transition is affected by the temperature and the total operating time of the display, and that compensating for these factors is desirable to secure accurate gray scale rendition.
As briefly mentioned above, it has been found that, at least in some cases, the impulse necessary for a given transition in a bistable electro-optic display varies with the residence time of a pixel in its optical state, this phenomenon hereinafter being referred to as “dwell time dependence” or “DTD”, although the term “dwell time sensitivity” was used in the aforementioned Application Ser. No. 60/320,070. Thus, it may be desirable or even in some cases in practice necessary, to vary the impulse applied for a given transition as a function of the dwell time of the pixel in its initial optical state.
The phenomenon of dwell time dependence will now be explained in more detail with reference to FIG. 1 of the accompanying drawings, which shows the reflectance of a pixel a function of time for a sequence of transitions denoted R3→R2→R1, where (generalizing the nomenclature used above) each of the Rk terms indicates a gray level in a sequence of gray levels, with R's with larger indices occurring before R's with smaller indices. The transitions between R3 and R2 and between R2 and R1 are also indicated. DTD is the variation of the final optical state R1 caused by variation in the time spent in the optical state R2, referred to as the dwell time. One can compensate for DTD by choosing different waveforms for different dwell times or different ranges of dwell times in the previous optical state. This method of compensation is called “dwell-time compensation,” “DTC”, or simply “time compensation”.
However, such DTC may conflict with other desirable properties of drive schemes. In particular, for reasons discussed in detail in the MEDEOD applications, with many electro-optic displays it is highly desirable to ensure that the drive scheme used is direct current (DC) balanced, in the sense that, for any arbitrary series of transitions beginning and ending in the same optical state, the applied impulse (i.e., the integral of the applied voltage with respect to time) is zero. This guarantees that the net impulse (also called “DC imbalance”) experienced by any pixel of the display is bounded by a known value regardless of the exact series of transitions undergone by that pixel. For example, a 15 V, 300 msec pulse may be used to drive a pixel from a white to a black state. After this transition, the pixel has experienced 4.5 V sec of DC imbalance impulse. If a −15 V, 300 msec pulse is used to drive the pixel back to white, then the pixel is DC balanced for the overall excursion from white to black and back to white. This DC balance should hold for all possible excursions from one original optical state, to a series of optical states the same as or different from the original optical state, then back to the original optical state.
A drive scheme can be dwell-time-compensated by adding or removing voltage features to or from a base drive scheme. For example, one might begin with a drive scheme for a two optical state (black and white) display, the drive scheme including the following four waveforms:
TABLE 1TransitionWaveformblack to black0 V for 420 msecblack to white−15 V for 400 msec, then 0 V for 20 msecwhite to black+15 V for 400 msec, then 0 V for 20 msecwhite to white0 V for 420 msec
This drive scheme is DC balanced, because any series of transitions that brings a pixel back to its initial optical state is DC balanced, that is, the net area under the voltage profile for the entire series of transitions is zero.
Optical errors can arise from DTD of a display. For example, a pixel may can start in the white state, drive to the black state, dwell for a time, and then drive back to the white state. The final white state reflectance is a function of the time spent in the black state.
It is desirable to have a very small DTD. If this is not possible for a specific electro-optic display, it is desirable to compensate for DTD, in accordance with one aspect of the present invention, by selecting different waveforms for different ranges of dwell times in the prior optical state. For example, one may find that the final white state in the example just given is brighter after short dwell times in the previous black state than after long dwell times in the previous black state. One dwell-time-compensation scheme would be to modify the duration of the pulse that brings the pixel layer from black to white to counteract this DTD of the final optical state. For example, one could shorten the pulse length in the black-to-white transition when the dwell time in the previous black state is short, and keep the pulse longer for long dwell times in the previous black state. This tends to produce a darker white state for shorter prior-state dwell times, which counteracts the effects of DTD. For example, one could choose a black-to-white waveform that varies with dwell time in the black state according to Table 2 below.
TABLE 2Dwell timeWaveform0 to 0.3 sec−15 V for 280 msec, 0 V for 140 msec0.3 sec to 1 sec−15 V for 340 msec, 0 V for 80 msec1 sec to 3 sec−15 V for 380 msec, 0 V for 40 msec3 sec or greater−15 V for 400 msec, 0 V for 20 msec
The problem with this approach to DTC of a drive scheme is that the drive scheme as a whole is no longer DC balanced. Because the impulse for a black-to-white transition is a function of the time spent in the black state, and similarly the impulse for a white-to-black transition may be a function of the dwell time in the white state, the net impulse over a black-to-white-to-black sequence is, in general, not DC balanced. For example, suppose this sequence is carried out with a black-to-white transition after a short dwell time in black using a voltage pulse of −15 V for 280 msec=−4.2 V sec impulse, followed, after a long dwell in the white state, by a white-to-black transition using a voltage pulse of 15 V for 400 msec, for an impulse of 6 V sec. The net impulse in this sequence (black-white-black loop) is −4.2 V sec+6 V sec=1.8 V sec. Repeating this loop causes a build-up of DC imbalance, which can be detrimental to the performance of the display.
Thus, this aspect of the present invention provides a method for dwell time compensation of a DC balanced waveform or drive scheme that preserves the DC balance of the waveform or drive scheme.
Another aspect of the present invention relates to methods and apparatus for driving electro-optic displays which permits rapid response to user input. The aforementioned MEDEOD applications describe several methods and controllers for driving electro-optic displays. Most of these methods and controllers make use of a memory having two image buffers, the first of which stores a first or initial image (present on the display at the beginning of a transition or rewriting of the display) and the second of which stores a final image, which it desired to place upon the display after the rewrite. The controller compares the initial and final images and, if they differ, applies to the various pixels of the display driving voltages which cause the pixels to undergo changes in optical state such that at the end of the rewrite (alternatively called an update) the final image is formed on the display.
However, in most of the aforementioned methods and controllers, the updating operation is “atomic” in the sense that once an update is started, the memory cannot accept any new image data until the update is complete. This causes difficulties when it is desired to use the display for applications that accept user input, for example via a keyboard or similar data input device, since the controller is not responsive to user input while an update is being effected. For electrophoretic media, in which the transition between the two extreme optical states may take several hundred milliseconds, this unresponsive period may vary from about 800 to about 1800 milliseconds, the majority of this period be attributable to the update cycle required by the electro-optic material. Although the duration of the unresponsive period may be reduced by removing some of the performance artefacts that increase update time, and by improving the speed of response of the electro-optic material, it is unlikely that such techniques alone will reduce the unresponsive period below about 500 milliseconds. This is still longer than is desirable for interactive applications, such example an electronic dictionary, where the user expects rapid response to user input. Accordingly, there is a need for an image updating method and controller with a reduced unresponsive period.
This aspect of the present invention makes use of the known concept of asynchronous image updating to reduce substantially the duration of the unresponsive period. It is known to use structures already developed for gray scale image displays to reduce the unresponsive period by up to 65 percent, as compared with prior art methods and controllers, with only modest increases in the complexity and memory requirements of the controller.
Finally, this invention relates to a method and apparatus for driving an electro-optic display in which the data used to define the drive scheme is compressed in a specific manner. The aforementioned MEDEOD applications describe methods and apparatus for driving electro-optic displays in which the data defining the drive scheme (or plurality of drive schemes) used are stored in one or more look-up tables (“LUT's”). Such LUT's must of course contain data defining the waveform for each waveform of the or each drive scheme, and a single waveform will typically require multiple bytes. As described in the MEDEOD applications, the LUT may have to take account of more than two optical states, together with adjustments for such factors as temperature, humidity, operating time of the medium etc. Thus, the amount of memory necessary for holding the waveform information can be substantial. It is desirable to reduce the amount of memory allocated to waveform information in order to reduce the cost of the display controller. A simple compression scheme that can be realistically accommodated in a display controller or host computer would be helpful in reducing the display controller cost. This invention relates to a simple compression scheme that appears particularly advantageous for electro-optic displays.